Issued by NWS North Platte, NE
000 FXUS63 KLBF 210906 AFDLBF AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE NORTH PLATTE NE 306 AM CST SAT NOV 21 2009 .SYNOPSIS... LONG WAVE TROF CRASHING ONSHORE ACROSS THE WEST COAST WITH WELL DEFINED WAA ZONE OUT AHEAD OF IT AND DEVELOPING MID AND UPPER LEVEL CLOUDS OVER THE CENTRAL ROCKIES INTO THE NRN PLAINS. LOWERING PRESSURES IN RESPONSE TO ELVOLVING SWRLY FLOW FURTHER WEST ALLOWING DECENT LLJ FOR THE TIME OF YEAR AND ASSOC MIXING ACROSS THE SANDHILLS KEEPING TEMPS UP THIS MORNING. && .SHORT TERM...(TODAY THROUGH TUESDAY) BRIEF TRANSITORY RIDGE EXISTS TODAY BEING REPLACED BY DEVELOPING LEE TROF AS LEAD SHORT WAVE PROGRESSES QUICKLY ACROSS SRN CANADA. SWRLY FLOW AS UPPER HIGH EXISTS ALLOWS EML TO OVERSPREAD THE AREA THIS MORNING AND ALTHOUGH MIXING MAY BE LIMITED INITIALLY DUE TO CLOUD COVER...AREAS EAST OF A LINE FROM OGA TO VTN WILL BENEFIT FROM MECHANICAL MIXING TO ALLOW TEMPS TO REACH U50S TO POSSIBLY L60S. AS SHORT WAVE CONTINUES ACROSS SRN CANADA TONIGHT...WEAK FRONT WILL PROGRESS THROUGH THE CWA OVERNIGHT. INITIALLY...MID LEVEL FGEN WILL ALLOW A COUPLED CIRCULATION TO KEEP FRONT MOVING EWD...BUT AS UPPER SUPPORT OUTRUNS SFC FEATURE FRONTOLYSIS AND EVENTUALLY LOW LEVEL FRONTAL FRACTURE OCCURS JUST SE OF CWA. DRY MID AND LOWER LAYERS WILL INHIBIT PREICP REACHING THE GROUND AND HAVE LEFT FCST DRY SAT NIGHT AND EARLY SUNDAY. LOW LEVEL MOISTURE POISED TO RETURN NWD SUNDAY MORNING...BUT SHOULD BE DEPLACED E OF ADVANCING FRONT AND THEREFORE E OF CWA AND HAVE REMOVED MENTION OF FOG IN ZONES. THIS WILL HAVE TO BE MONITORED IF FRONT WEAKENS EARLY. NEXT...MORE IMPRESSIVE...SHORT WAVE BEGINS TO DIG ACROSS THE CENTRAL ROCKIES SUNDAY. LEE TROF IN ADVANCE OF THIS WILL ALLOW WINDS TO SHIFT TO SRLY AGAIN AND WILL BEGIN THE TRANSPORT OF MOISTURE NWWD. SUNDAY WILL BE COOLER BEHIND THE FRONT. THE MOISTURE WILL HAVE A HARD TIME INITIALLY MOVING TOO FAR W AND DRY MID LEVELS WILL INHIBIT ANY PRECIP DEVELOPMENT THROUGH SUNDAY NIGHT. LOW LEVELS WILL SATURATE SUNDAY NIGHT AND BR AND STRATUS WILL LIKELY BE THE RESULT. LOW TEMPS HAVE BEEN ADJUSTED SLIGHTLY UPWARD TO ACCOUNT FOR INCREASE OF LL MOISTURE AND CLOUD COVER. THOUGH DECENT SUNSHINE FOR SUNDAY...SERLY WINDS WILL BE RECYCLING COOLER POST FRONTAL AIRMASS. SIGNIFICANT DIFFS IN MODELS BY EARLY MONDAY. 00Z 21 NOV RUN OF THE ECMWF WOULD HAVE THE MOST IMPACT FOR THE CWA...BUT THIS MODEL HAS DRIFTED NWD WITH THE UPPER LOW COMPARED TO ITS 12Z 20 NOV COUNTERPART. AT THE SAME TIME...THE GFS HAS TRENDED SLIGHTLY SLOWER AND SWD...BUT WOULD LEAVE WRN CWA MAINLY DRY AND PRECIP FOR NERN CWA WOULD COME DURING THE DAY MONDAY. EVEN IF THE ECMWF VERIFIES...A LOOK AT MOISTURE TRANSPORT VECTORS ON ISENTROPIC SFCS WOULD SUGGEST WCB WOULD NOT HAVE A CHANCE TO WORK INTO THE SYSTEM UNTIL IT REACHES ERN NEBR. THERE DOES APPEAR TO BE A SMALL WINDOW OF OPPORTUNITY THOUGH FOR WCB/CCB INTERACTION AND ASSOCIATED FGEN IN FAVORED NW QUAD OF DEVELOPING UPPER LOW...OVER NERN CWA BEGINNING MONDAY NIGHT AND CONTINUING TUESDAY. IN ADDITION TO AND AS A RESULT OF TIMING DIFFS BTWN MODELS...PRECIP TYPE IS AT ISSUE AS WELL. WARM LOW LEVELS AS SUGGESTED BY BUFR SOUNDINGS WOULD INDICATE LITTLE CHANCE FOR SIGNIFICANT ACCUMULATING SNOWS UNTIL TUES...AND ONLY IF ECMWF VERIFIES. GFS DOES BRING MORE COLD AIR SWD THAN ECMWF ON MONDAY AND THEREFORE SNOWFALL WOULD START SOONER...BUT WOULD END SOONER AS WELL. TOO MANY DIFFS AT THIS POINT TO WORK OUT DETAILS UNFORTUNATELY. .LONG TERM...(WEDNESDAY THROUGH FRIDAY) EARLIER DISCUSSED DIFFS MAGNIFY LATER IN THE WEEK LEADING TO A WIDE ENVELOPE OF PLAUSIBLE SOLUTIONS. HOWEVER END RESULT BY THE END OF THE WEEK WOULD BE TO TREND TWD INCREASING HEIGHTS ALOFT AND A DRY FCST. NO CHANGES MADE BEYOND MON/TUE TIME FRAME FOR NOW...BUT EARLY INDICATIONS FOR THANKSGIVING ARE MAINLY DRY...THOUGH TRENDING MORE ACTIVE TWD THE WEEKEND. && .AVIATION... A SURFACE TROUGH OF LOW PRESSURE WILL DEEPEN AND TRACK EAST TODAY ACROSS WESTERN AND NORTH CENTRAL NEBRASKA. STRONG SOUTHERLY WINDS OF 15 TO 25 KTS WILL DEVELOP EAST OF THIS FEATURE...WHILE WESTERLY WINDS OF 10 TO 20 KTS WILL BE COMMON WEST OF THE SURFACE TROUGH. WINDS ARE EXPECTED TO DIMINISH THIS EVENING WITH WESTERLY WINDS OF 5 TO 15 KTS EXPECTED. MID TO HIGH LEVEL CLOUDINESS WILL BE ON THE INCREASE THIS AFTERNOON AND TONIGHT EAST OF AN APPROACHING UPPER LEVEL TROUGH OF LOW PRESSURE. CEILINGS AOA 15000 FEET AGL ARE LIKELY AT BOTH THE KVTN AND KLBF TERMINALS THROUGH 12Z SUNDAY. && .LBF WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM/EXTENDED...JWS AVIATION...CLB
